This invention relates generally to semiconductor packaging and specifically to a method for fabricating plastic semiconductor packages with reduced package bow.
BACKGROUND OF THE INVENTION
A conventional plastic semiconductor package includes a semiconductor die encapsulated in a molded plastic body. The molded plastic body rigidities and protects the die from the environment. A plastic semiconductor package also includes a metal leadframe wire bonded to bond pads on the die. An adhesive member, such as polyimide tape, or alternately an adhesive layer on the lead frame, attaches the die to the leadframe. The leadframe forms terminal leads for the package and provides internal signal, power and ground paths through the package body to the die.
One aspect of a plastic semiconductor package is that the molded plastic body, the die, the leadframe and the adhesive tape comprise different materials, having different coefficients of thermal expansion (CTE). Because of the different coefficients of thermal expansion, thermo-mechanical stresses are generated within the package as the package is subjected to a temperature change. These stresses are particularly large during manufacture, as the package body cools from a relatively high molding temperature (e.g., 183xc2x0 C.) to room temperature (e.g., 25xc2x0 C.).
One problem that results from these thermo-mechanical stresses is referred to as xe2x80x9cpackage bowxe2x80x9d or xe2x80x9cpackage warpagexe2x80x9d. FIGS. 1A-1C illustrate different types of package bow, or warpage, in plastic semiconductor packages. In FIG. 1A, a semiconductor package 10A includes a plastic body 12A and a plurality of terminal leads 14A extending from the body 12A. on opposed longitudinal sides thereof. The package 10A has undergone a negative bow and has the profile of a xe2x80x9cfrownxe2x80x9d. Specifically, the package body 12A has bowed with respect to a theoretical flat profile FP by a warp distance X, which by convention is designated (xe2x88x92). In addition, some of the terminal leads 14A at the center of the package 10A are offset from a theoretical planar reference plane RP by an offset distance of Y.
In FIG. 1B, a semiconductor package 10B includes a package body 12B which has undergone a positive bow, and has the profile of a xe2x80x9csmilexe2x80x9d. Specifically, the package body 12B has bowed with respect to the theoretical flat profile FP by the warp distance X, which by convention is designated (+). In addition, some of the terminal leads 14B at the ends of the package 10B are offset from the theoretical planar reference plane RP by the offset distance Y.
In FIG. 1C, a semiconductor package 10C includes a package body 12C which has undergone both negative and positive warp distances X, and has an xe2x80x9cundulatingxe2x80x9d profile. In addition, some of the terminal leads 14C in the center of the package 10C, as well as some of the terminal leads 14C at an end of the package 10C, are offset from the theoretical reference plane RP by the offset distance Y.
In each of the packages 10A, 10B, 10C the package bow has been illustrated as occurring along a longitudinal axis of the package. However, package bow can occur along any axis (e.g., lateral axis, diagonal axis) of the package 10A, 10B, 10C. Still further, package bow can be evaluated at any point on the package 10A, 10B, 10C. For example, one method for evaluating package bow is to measure the warp distance X at many different points on a major surface (e.g., top surface or bottom surface) of the package 10A, 10B, 10C. These measurements can be made using a surface laser profiler, a surface roughness meter, or with other conventional instruments known in the art.
Package bow can also be defined by a xe2x80x9cwarp factorxe2x80x9d. SEMI (Semiconductor Equipment and Materials International) defines the warp factor as the warp distance X in mils divided by the length of the package in inches (i.e., WF=X(mils)/L(inches). SEMI standards (SEMI G37-88) for plastic molded small outline packages specify an acceptable warp factor as being 2.5 or less. SEMI standards (SEMI G37-88) also define package warpage as any non-linear dimensional change from the mold cavity characteristic.
One problem resulting from package bow is that the terminal leads 14A, 14B, 14C of the package are no longer co-planar. Without co-planar terminal leads 14A, 14B, 14C, surface mounting of the package 10A, 10B, 10C to a supporting substrate, such as a circuit board, can be difficult. SEMI standards (SEMI G37-88) for plastic molded small outline packages specify that lead co-planarity, measured in a vertical direction, must be within 3 mils (i.e., Y less than 3 mils). For making planarity measurements, the reference plane RP can be defined by the three lowest terminal leads 14A, 14B, 14C from the bottom of the package 10A, 10B, 10C. In addition to affecting lead planarity, in lead on chip packages (LOC), package bow can adversely affect the planarity of the leadfingers on the leadframe, and the adhesive bonds to the die.
Also with package bow, once the package is surface mounted to the supporting substrate, additional stresses are generated at the soldered connections between the terminal leads 14A, 14B, 14C and the supporting substrate. These stresses can cause solder joint failure, and can decrease the lifetime of the package 10A, 10B, 10C. Package bow can also cause problems during handling of the packages 10A, 10B, 10C by automated pick and place equipment, which requires planar surfaces for suction cups to operate properly.
Package bow is particularly troublesome in thin packages, such as thin small outline packages (TSOP). In addition, package bow has become more of a problem due to decreases in the thicknesses of semiconductor dice. Conventional semiconductor dice, for example, have recently decreased in thickness from about 28 mils to about 14 mils. The thinner dice are more likely to bow in a package, and are less likely to provide a rigidifying structure in the package capable of resisting thermo-mechanical stresses.
In view of the foregoing, improved plastic semiconductor packages able to resist bowing, and improved methods for fabricating plastic semiconductor package with reduced bowing, are needed in the art.
In accordance with the present invention, a bow resistant plastic semiconductor package is provided. Also provided are a method for fabricating plastic semiconductor packages with reduced package bow, and improved electronic assemblies fabricated using the plastic semiconductor package.
The plastic semiconductor package includes a die, a lead frame attached to the die, and a plastic body encapsulating the die and the lead frame. In an illustrative embodiment, the package has the configuration of a thin small outline package (TSOP) having terminal leads in a gull wing configuration. Also in the illustrative embodiment, the lead frame and the die have a LOC (lead-on-chip) configuration, and an adhesive member attaches the die to lead fingers on the lead frame. In addition, wire bonds are formed between bond pads on the die, and the lead fingers on the leadframe, to provide electrical paths therebetween.
The plastic body includes a molded inner member that encapsulates the die, the wire bonds, and portions of the lead fingers proximate to the die. The plastic body also includes a molded outer member, that encapsulates the inner member, and portions of the lead fingers proximate to the terminal leads. The molded inner member provides additional structural strength and rigidity for the package. In addition, the molded inner member is dimensioned to equalize the volume of molding compound in the molded outer member on either side of the leadframe. Stated differently, the molded outer member comprises substantially equal volumes of molding compound on either side of a package parting line.
With the molded outer member comprising substantially equal volumes of molding compound, the magnitude of thermo-mechanical stresses developed as the molding compound undergoes shrinkage during cooling are decreased. The package body is therefore less likely to bow, and cause the terminal leads and the lead fingers to become non-planar. Further, the molded inner member can be at least partially cured prior to molding of the outer member, such that the package body has an internal rigidity for resisting package bow during cooling of the outer member. The molded inner member can also be configured to provide increased structural rigidity for surface mounting the package in electronic assemblies.
The package fabrication method can be performed using a two stage encapsulation process including: a first encapsulation stage for molding the inner member over the die and first portions of the lead fingers, and a second encapsulation stage for molding the outer member over the inner member and second portions of the lead fingers. The two stage encapsulation process can be performed using separate mold cavities having required geometries, and using the same molding compound for each stage. In addition, the inner member can be at least partially cured prior to molding of the outer member on the inner member.
In an alternate embodiment package, the inner member does not encapsulate the die, but comprises one or more volume equalizing members attached to the lead frame, and encapsulated in the plastic body. The volume equalizing members can comprise an electrically insulating material such as a molding compound, glob top material, or polymer tape. In addition to providing structural strength and rigidity for the package, the volume equalizing members are dimensioned to equalize the volume of molding compound above and below the package parting line.
The bow resistant package improves the construction of electronic assemblies such as printed circuit boards, and multi chip modules. In particular, stresses on bonded connections between the package leads and electrodes on a substrate of the assembly are substantially eliminated due to the planarity of the package leads.